1. Field of the Invention
The present invention relates to a semiconductor manufacturing apparatus, and particularly to a semiconductor manufacturing apparatus in which various processes such as oxidation, diffusion, and thermal treatment are performed for semiconductor wafers.
2. Description of the Related Art
The manufacture of semiconductors includes a process of forming a thermal oxidation film on the surface of a silicon wafer. This process is performed using, for example, a vertical-type semiconductor manufacturing apparatus shown in FIGS. 1A and 1B.
FIG. 1A shows a cross-section of the conventional semiconductor manufacturing apparatus 1. FIG. 1B shows a cross-section taken along a line X1--X1 of FIG. 1A.
As shown in FIGS. 1A and 1B, in the semiconductor manufacturing apparatus 1, a liner tube 230 is disposed within a hollow heater 220, and a reaction tube 100 is disposed within the liner tube 230. The liner tube 230 is formed of a material having a large thermal capacity and its function is to provide a uniform temperature distribution within a furnace. The reaction tube 100 has a reaction tube body 10 having a circular cross-section and a single gas feed pipe 20. An upper gas feed pipe 40 is disposed on a ceiling plate 14 of the reaction tube body 10. The upper gas feed pipe 40 and the ceiling plate 14 define a shower chamber 42. A portion of the ceiling plate 14 located within the shower chamber 42 serves as a gas shower plate 30 having a plurality of gas diffusion holes 32. The shower chamber 42 communicates with the interior of the reaction tube body 10 through the gas shower plate 30. The gas feed pipe 20 is vertically provided along a generatrix of the reaction tube body 10 and on the outer surface of the reaction tube body 10. A gas inlet portion 21 is provided at the lower end portion (upstream-side end portion) of the gas feed pipe 20, and the upper end portion (downstream-side end portion) thereof communicates with the shower chamber 42 located at the top portion of the reaction tube body 10. An exhaust pipe 50 is provided adjacent to the lower end of the reaction tube body 10 such that it communicates with the reaction tube body 10. The gas inlet portion 21 is connected to a reaction gas source (not shown), and the exhaust pipe 50 is connected to an exhauster (not shown).
A boat 210 is raised and lowered by a boat elevator (not shown), thereby introducing the boat 210 into the reaction tube body 10 or removing the boat 210 therefrom. The boat 210 stands on a boat cap 60, which is provided on a furnace cover 62. A flange 12 is provided along the periphery of the reaction tube body 10 at the lowermost end thereof. An O-ring 64 is interposed between the flange 12 and the furnace cover 62, thereby maintaining the reaction tube body 10 in an airtight state.
The boat 210 has four boat support shafts 212, on which a plurality of wafers 200 are loaded in a horizontal posture such that one wafer is located above another. Each wafer 200 loaded in the boat 210 undergoes oxidation to form an oxide film on the surface thereof.
For oxidation of the wafers 200, while the interior of the furnace and the wafers 200 are heated to and maintained at an oxidation temperature, the oxygen gas is fed to the shower chamber 42 through the gas feed pipe 20 and then diffused into the reaction tube body 10 through the gas diffusion holes 32 formed in the gas shower plate 30. The oxygen gas reacts with the wafers 200 to form oxide films on the wafer surfaces. The gas after reaction is discharged through the exhaust pipe 50.
The length of the reaction gas feed pipe 20 is substantially identical to that of the portion of the reaction tube body 10 located within the heater 220 and within the liner tube 230. Conceptually, oxygen gas is heated in the following manner. Oxygen gas introduced from the gas inlet portion 21 at around room temperature is heated while passing through the gas feed pipe 20. When the oxygen gas reaches the shower chamber 42, it reaches a temperature substantially equal to that of the inside of the reaction tube body 10 and the wafers 200 contained therein. The thus heated oxygen gas enters the reaction tube body 10.
However, since the oxygen gas fed from the gas inlet portion 21 has a temperature close to the room temperature, the temperature difference between the gas feed pipe 20 and the interior of the furnace increases toward the upstream-side portion of the gas feed pipe 20 and the amount of heat absorbed from the interior of the furnace increases accordingly. Hence, when the oxygen gas is fed through a single gas feed pipe 20, the upstream-side portion of the gas feed pipe 20 causes cooling in the portions of the wafers 200 located near the gas feed pipe 20. Particularly, this causes nonuniform temperature distribution over the wafer surface. As a result, since the wafer temperature affects the growth rate of oxide film, oxide film is formed having a nonuniform thickness over the wafer surface.